Directional couplers are passive devices used to couple part of the transmission power in one transmission line to a second transmission line. This is accomplished by locating a portion of the second transmission line close enough to the first transmission line that the electromagnetic signal passing through the first transmission line is electromagnetically coupled to the second transmission line. As shown in the schematic representation of FIG. 1, a typical directional coupler comprises first and second transmission lines 10, 20 that are electromagnetically coupled in a coupling region 25 and four ports 30, 40, 50, 60, one on each end of the coupling region of the transmission lines. By convention, the input and output ports of the signal transmission line (line 10 in FIG. 1) are referred to as the input and direct (or transmitted) ports and these are labeled ports 30 and 40, respectively, in FIG. 1. The ports of the coupled transmission line (line 20 in FIG. 1 are referred to as the coupled and isolated ports and these are labeled ports 50 and 60, respectively, in FIG. 1.
The coupling between the first signal transmission line 10 and the coupled transmission line 20 is ordinarily measured by a coupling factor in units of deciBels (dB). The coupling factor is defined as:coupling factor (dB)=10 log Pout/Pinwhere Pin is the input power at port 30 and Pout is the output power at port 50. For example, if half the power is coupled from the first transmission line 10 to the second transmission line 20, the coupling factor is −3 dB.
Ideally, electromagnetic coupling between the two transmission lines occurs over a distance that is a quarter wavelength (λ/4) of the signal being transmitted on the transmission line. However, over a considerable part of the operating frequency range (20 MHz to 40 GHz) of electromagnetic signals on transmission lines, the wavelength of the signal is too large to permit the practical use of a directional coupler that is λ/4 long. For example, at 1 GHz, the wavelength is approximately 1 foot in length and at 100 MHz it is 10 feet in length. In such circumstances, the coupling distance in practical devices is typically a fraction of the ideal λ/4.
Unfortunately, the coupling factor is a function of frequency and the variation with frequency is exacerbated by the departure from ideal conditions. A typical plot of signal coupling in dB versus frequency is set forth in FIG. 2. As can be seen, the coupling factor ranges from about −24 dB at 100 MHz to about −12.5 dB at 500 MHz.
For many applications, this amount of variation in the coupling factor is undesirable and, as a practical matter, the only alternative is to limit the bandwidth of the coupler to a narrow enough range that the variation in coupling factor is acceptable. As a practical matter, this requires that conventional couplers have a bandwidth that is no more than about 50% of their center frequency.